Signaling through the B cell receptor (BCR) is critical for peripheral B cell maturation. Immature Ig M (BCR)-positive B cells emigrate from the bone marrow to the spleen. Once there, immature early transitional (T1) B cells are thought to sequentially differentiate into late transitional (T2) B cells and then into mature follicular B (FoB) cells. We have previously shown that T1 and T2 cells respond differently to BCR cross-linking in vitro; T1 cells die while T2 cells survive, proliferate and differentiate into mature FoB cells. We have recently found that BCR-mediated DAG and IP3 generation occurs preferentially in T2 compared to T1 B cells, yet both subsets mobilize calcium. BTK- and PLC-gamma2- deficient B cells, which do not produce DAG, are developmentally blocked at the T2 stage, suggesting that DAG-mediated signals are essential for the differentiation of T2 into mature FoB cells. RasGRP3, a downstream effector of DAG, functions to activate Ras; in B cell lines, RasGRP3 has been shown to be essential for BCR-mediated Ras activation. As Ras signaling has been implicated in lymphocyte survival and differentiation, I hypothesize that DAG promotes the survival and differentiation of T2 B cells via RasGRP3/Ras pathway. To test this hypothesis, I will analyze peripheral B cell development in rasgrp3-/- mice. In addition, I will investigate the role of RasGRP3 in B cell survival, differentiation, and activation in vitro and in vivo. The proposed experiments will reveal the significance of the DAG-dependent RasGRP3/Ras pathway in the development and function of mature FoB cells. [unreadable] [unreadable] [unreadable]